Erasable inks containing thermoplastic block copolymers

ABSTRACT

An erasable ink containing a thermoplastic block copolymer, pigment particles, and a solvent. In a preferred embodiment, an erasable ink requiring little or no pressurization is provided by employing a mixture of radial and linear block copolymers and a mixed solvent system falling within a specific range of solubility parameter. Polybutene and poly-alpha-methylstyrene are preferred additives to decrease smearing tendency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an erasable ink, and more particularly,to an erasable ink containing a thermoplastic block copolymer.

2. Description of the Prior Art

In choosing a writing instrument, line intensity and erasability must betaken into account. Until recently, the choice was between these twofactors since it was impossible to have both high line intensity andhigh erasability in the same writing instrument. Writing instrumentssuch as pencils possess high erasability (i.e., the written line can beremoved readily) but are relatively low in line intensity (i.e., thecontrast between the written line and the paper is low). Because ofthis, a xerographic copy of a document written in pencil is typically apoor reproduction. In contrast, writing instruments such as ball pointpens, fountain pens, and soft- or porous-type pens produce written lineswhich are relatively intense but lack any significant degree oferasability without damage to the underlying writing surface, or requireadditional material to mask the unwanted lines.

Accordingly, the prior art contains numerous attempts at writinginstruments containing inks which produce a written line of highintensity, and yet are erasable for at least the first few hours afterbeing placed upon the writing surface. This combination produces awritten document having all the desirable attributes of both systems,and within a short period of time being written, possess the permanencytypically desired in written documents.

Once such system is described in U.S. Pat. No. 3,875,105 to Daugherty etal., issued on Apr. 1, 1975. That patent describes an erasable writingmedium suitable for use in ball point pens having a discontinuous phaseof a solid colorant and a homogeneous continuous phase including amatrix material having cohesive properties exceeding its property toadhere to the substratum being written upon. The ink consists of apigment, stabilized with adsorbed surfactant, dispersed in amulticomponent solvent system containing a polyvinyl methyl etherelastomer. On transfer to paper, the low boiling toluene evaporates intothe air or penetrates into the paper causing a viscosity increase andenhancement of the elastic characteristics of the pigment dispersion.Adhesion between the elastomer and pigment remains greater than thatbetween the elastomer and paper until certain components of the solventsystem migrate into the paper, at which time polymer adhesion increasesand erasability decreases. The erasability of the ink is stated todepend in part on the rapid evaporation of at least one of thecomponents of the continuous phase. Accordingly, the volatile componentis stated to be one preferably with an evaporation rate of 3 to 15 on arelative numerical scale on which ethyl ether is assigned an evaporationrate of 1, such as toluene which is stated to have a rate of 6.1. Thevolatile component is employed in a range from 24.0 to 29.0%. The lineintensity of that ink is dependent upon the relative quantity of pigmentused. As stated in that patent, it may be necessary to dispense the inkin a pressurized ball pen depending upon the relative quantity ofpigment used since at higher levels of pigment the ink may not flow withsufficient rapidity through the feed passageways of the conventional,gravity-fed ballpoint pen. Moreover, in an ink of such a high level ofvolatile component, some means must be provided to prevent the volatilecomponent from evaporating into the atmosphere which would increase theviscosity of the ink while in the pen.

U.S. Pat. No. 4,097,290 to Muller et al., incorporated herein byreference, describes a ballpoint writing instrument capable of writingwith an intense line which is easily erasable by mechanical means for aninitial period of several hours but eventually becomes non-erasable.These properties are attained by an ink composition containing certainspecific rubbers and volatile solvents which control erasability. Therubbers employed are natural or of a chemical structure essentiallyduplicating that of natural rubber. In addition, the ink contains avolatile low boiling organic solvent having a boiling point less than180° C. and exhibiting 100% evaporation within 60 minutes, and a highboiling organic liquid solvent having a boiling point of greater than300° C. The volatile low boiling organic solvent is included to rapidlyincrease the viscosity of the ink in the written line thereby minimizingpenetration by the ink into the paper. Although the ink yields a veryintense line with good erasability for the first few hours after it iswritten, it suffers from the same disadvantage as that set forthpreviously for the Dougherty et al. Patent. Namely, it possesses arelatively high concentration of a low boiling, rapidly vaporizablesolvent, and a relatively high concentration of pigment such that meansfor subjecting the ink to super-atmospheric pressure is necessary tofacilitate the flow and supply of ink to the ball. In addition, therubber must be milled prior to its incorporation into the ink to producean average molecular weight of between 100,000 and 900,000 andpreferably between 400,000 and 750,000. The milling process is somewhatdifficult to control in that milling too rapidly, or for too long atime, will produce rubber particles unsuitable for use in the ink.

There are often also problems in addition to time and expense when inkmust be supplied under pressure. If the ball doesn't fit almostperfectly in the ball seat, point bleed can occur, and will become worseas the ball seat wears with writing. Writing smoothness is also affectedby pressurization because the ball must be pushed into its seat, and theink pressure must be counteracted in order to get ink flow. Anothermajor disadvantage of prior art erasable ink is that the written line iseasily smeared.

Accordingly, a need exists for an erasable ink for use in ballpoint penswhich will produce a line of high erasability and high line intensity,which does not depend upon a high level of volatile solvent, does notsmear, is readily prepared without the necessity of controlled millingof the rubber component, and requires little or no pressurization.

SUMMARY OF THE INVENTION

The present invention comprises an erasable ink composition containing athermoplastic block copolymer, a pigment, and a solvent for thecopolymer.

DETAILED DESCRIPTION OF THE INVENTION

It has now been found that an erasable ink producing a written line ofhigh line intensity and high erasability can be produced withoutemploying a high volatility solvent or high levels of pressurization, bythe use of thermoplastic block copolymers in place of the matrixmaterial of Daugherty et al. or the rubbers of Muller et al.

The block copolymers of the present invention are best described asthermoplastic elastomers which have an ABA, (AB)_(n) X, or (--AB)--_(n)arrangement of A (thermoplastic) and B (rubbery) blocks. The hard Ablocks may be glassy (e.g., polystyrene) or crystalline (e.g.,polyester, polyurethane); the soft B blocks must be elastomeric (e.g.,polybutadiene, polyisoprene). When the hard segments are incompatiblewith the soft segments, the domains or regions of hard blocks act asreinforcing physical cross-links for the rubbery matrix. In contrast tochemically cross-linked rubbers, the physical network is thermallyreversible. When the polymer is heated above the T_(g) (or T_(m)) of thehard block, the hard blocks soften and allow the rubber to flow and tobe processed as a thermoplastic.

The manufacture of block copolymer thermoplastic elastomers depends uponthe type and arrangement of the blocks. For example, styrene-butadieneABA and (AB)_(n) X block copolymers are conveniently prepared byalkylithium initiated anionic polymerization. Thermoplastic --A-B--_(n)polyurethanes are synthesized by step-growth addition copolymerizationof dihydroxy compounds such as polytetramethylene ether glycol andtoluene diisocyanate. The copolyester-ether --A-B--_(n) copolymers areproduced by the polycondensation of dicarboxylic acids (e.g.,terephthalic acid) with glycols or polyether glycols. The preferredblock copolymers of the present invention are those made fromstyrene-diene comonomers having a block arrangement of ABA or (AB)_(n) Xsuch as those sold under the trade names Kraton (Shell Chemical) andSolprene (Phillips Chemical Company).

A preferred block copolymer for use in the present invention is astyrene-isoprene-styrene block copolymer. Each molecule consists of anindividual chain of three blocks--an elastomeric block in the center(isoprene) and a thermoplastic block on each end (styrene). Thepolystyrene end blocks form a discrete phase which locks the continuousphase isoprene midblocks into a elastomeric network. Because mechanicalcross-links rather than chemical cross-links are present, the copolymerscan be dissolved in many common hydrocarbon solvents withoutpremastication. By dissolving the polystyrene domains, the network isunlocked permitting mixing with other materials. Since such rubbers arelow in molecular weight (typically less than 100,000 to about 150,000),the solutions produced have low viscosity even with high solids content.

Another type of preferred elastomer is a "radial" (AB)_(n) X blockcopolymer consisting of styrene and isoprene blocks. In a lineararrangement such as that described above, each molecule contains twostyrene blocks at the ends of an isoprene midblock. In contrast, thepreferred radial structure contains four isoprene midblocks which areall joined at a common center, with a styrene block at the outer end ofeach isoprene block. Compared to linear block copolymers, generally theradial block copolymers at similar molecular weight and monomer ratioproduce lower solution viscosities. For example, the viscosity of a 20%and 25% by weight solution in toluene of a linear polymer having amolecular weight of about 150,000 is 600 centipoises and 1600 centiposesrespectively. In contrast, solutions of a radial polymer having amolecular weight of between 175,000 and 275,000 at the sameconcentration in toluene have viscosities of 570 centipoises and 1650centipoises respectively. The typical range of molecular weights ofradial polymers is about 150,000 to about 300,000.

While the use of either linear or radial block copolymers alone resultsin an acceptable ink, it is preferred to blend the two together in arange of radial to linear of about 7:1 to 1:1. The preferred blending isin a ratio of about 3 to 1 radial to linear.

Block copolymers having butadiene or isoprene midblocks are preferred,although block copolymers having ethylene-butylene midblocks can also beused in pressurized systems. The range of concentration for the blockcopolymer is between about 10% and about 30% by weight. Higherconcentrations can be used but contribute little to erasability andresult in rapidly increasing viscosity with a concomitant increase inpressurization requirements. The ratio of diene to styrene is typicallybetween about 60:40 and about 90:10.

The molecular weight of the rubbers employed in the present invention islow, typically less than 300,000, as compared to general purpose rubberssuch as styrene-butadiene rubber, polysioprene rubber, and naturalrubber even after natural rubber is broken down by milling. Naturalrubbers used in the prior art typically have molecular weights betweenabout 400,000 and about 750,000. With block copolymers, breakdown isneither desired nor required. Instead, the polystyrene domains aresoftened by solvating which permits the polymer to flow under shear.Dissolution and solvent release are quite rapid with such blockcopolymers. They dissolve or disperse readily in common hydrocarbonsolvents to yield high solids, low viscosity solutions. Depending uponwhich solvent is used, the interaction between hard (glassy) and soft(rubbery) segments of a thermoplastic block will be affected. This inturn strongly influences the viscoelastic behavior of the rubber.

Three general factors must be considered in preparing inks according tothe present invention: solubility parameter, volatility, and criticalpigment concentration (CPC). These factors require greater considerationwhen preparing a pen needing low levels (less than 1.0 psi) or nopressurization. Each of these factors will be discussed generally beforefocusing on how each affects pressurization requirements.

In determining which solvents to employ in the ink of the presentinvention, the solubility parameter (represented by δ), which is ameasure of the total forces holding the molecules of a solid or a liquidtogether, is a useful guide. It has the units of (cal/cm³)^(1/2). Everycompound is characterized by a specific value of solubility parameter.Materials having about the same solubility parameter tend to formhomogeneous mixtures or to be miscible. Those with different solubilityparameters tend to form separate layers or to be mutually insoluble.Discussions of solubility parameter concepts are presented in (1)Encyclopedia of Polymer Science and Technology, Interscience, New York(1965), Vol. 3, Pg. 833; (2) Encyclopedia of Chemical Technology,Interscience, New York (1971), Sup. Vol., Pg. 889; (3) Polymer Handbook,J. Brandrup and E. H. Immergut, Interscience, New York (1966), pgs.IV-337 to IV-341; and (4) I. T. Smith, Review of Current Literature onthe Paint and Allied Industries, Vol. XXXVI, No. 247, (January 1963),Pgs. 1-4; all incorporated herein by reference.

The block copolymers in the present invention will dissolve in mostorganic solvents having solubility parameters between about 7 and about10.5. In the case of inks that will be subject to pressurization, lowboiling, high volatility solvents such as toluene, naphtha, and theother low boiling solvents in the Muller et al. Patent can beadvantageously employed. In such an embodiment, the solubility parameterof the organic solvent combination need only fall within the outerlimits of the solubility parameter of about 7 to about 10.5.

It will often be desirable to use a combination of solvents in the inkof the present invention since the resulting mixed solvent system actsas a solvent for the polymer, as a dispersant for the pigment, and as aplasticizer for the polymer or polymers. The formula to use indetermining the solubility parameter for such a mixed solvent system is##EQU1## The value for a particular solvent is readily obtained byconsulting one of the reference texts incorporated by reference above.

To give sufficient intensity to the written line produced by the ink ofthe present invention, finely divided pigmenting materials are added. Byfinely divided, reference is made to particle sizes within the range ofabout 0.01 microns to about 5 microns, the lower figure being alimitation as to visibility and the upper figure being limited byballpoint clearance; that is, the gap between the ball and the retaininglip of the socket.

The pigment is typically added to the other components of the ink in theform of a dispersion containing 50-60% of dioctyl phthalate. A typicalblue ink would contain 12.6% Victoria Blue having a particle size ofabout 0.025 microns and about 5.7% Phthalocyanine Blue having a particlesize of about 0.015 microns. Unless an unpressurized ink is desired asdiscussed below, the dispersion would then be diluted with an equalvolume of toluene or other similar solvent.

For every pigment/solvent system, as well as for everypigment/solvent/polymer system, there is a "critical pigmentconcentration" (CPC), below which no pressure is required to initiateflow. Above the CPC, constant pressure is required for pigment/solventsystems and the pressurization of pigment/solvent/polymer systemsincreases in proportion to increases in pigment concentration,signifying a pigment-polymer interaction. Each particular pigment,polymer and solvent combination has its own CPC. Thus it is difficult topredict what the CPC for any particular system will be.

For inks requiring no pressurization, low volatility solvents areemployed. Low volatility solvents are defined as those having a value ofnot greater than 10 on a scale of 0 to 100 where butyl acetate is usedas a standard and has a value of 100 as measured according to theNational Printing Ink Research Institute Raw Materials Handbook, Volume1, Organic Solvents, incorporated hereby by reference. Page 9 of thatvolume provides data for various solvents.

To produce a pen requiring a low level (less than 1.0 psi) or nopressurization, each of the above factors must be controlled within arelatively narrow range, and care must be excerised in choosing theblock copolymer or copolymers. Each of these factors will now bediscussed in connection with refills requiring little or nopressurization.

Block copolymers having butadiene or isoprene midblocks, and mixtures ofradial and linear block copolymers are preferred when preparing refillsrequiring little or no pressurization. For gravity flow with nopressurization, such block copolymers must be used in conjunction withan organic solvent or a mixture of organic solvents having a solubilityparameter between about 8.1 and about 8.7. Where no pressurization isinvolved, such solvent or solvents must also possess low volatility asdefined above. Preferred low volatility solvents for use in inks of thepresent invention are set forth in Table 1 below. One or more solventsfrom Group 1 are mixed with one or more solvents from Group 2 to producea combination having a solubility parameter in the appropriate range.

Finally, the CPC must be considered. It has generally been found thatthe CPC of most pigment/solvents/polymer systems is 9-10% or less.Choosing the exact pigment concentration is readily accomplished by oneskilled in the art after selecting a mixed solvent system and aparticular block copolymer.

                                      TABLE 1                                     __________________________________________________________________________    Solvent &                                                                              Chemical       Solubility                                                                          Boiling                                                                             Evaporation                               Manufacturer                                                                           Description    Parameter                                                                           Point, °C.                                                                   Rate.sup.a                                __________________________________________________________________________    GROUP 1                                                                       Isopar M Heavy isoparaffinic naphtha                                                                  7.2   207   <10                                       (Exxon)  --C.sub.11 -C.sub.17                                                 MagieSol 47 Oil                                                               (Magie)  Hydrotreated middle distillate                                                               7.2   238   <1                                        1102 Oil                                                                      (Witco)  Light vacuum gas oil                                                                         7.2   266   <1                                        MagieSol 44 Oil                                                               (Magie)  Hydrotreated middle distillate                                                               7.5   226   2                                         1108 Oil                                                                      (Witco)  Hydrotreated middle distillate                                                               7.3   280   <1                                        Norpar 13                                                                              Normal paraffin                                                                              7.4   229   <10                                       (Exxon)  --C.sub.11 -C.sub.17                                                 Dodecane C.sub.12 hydrocarbon                                                                         7.8   216   med.low                                   __________________________________________________________________________    GROUP 2                                                                       Aromatic 150                                                                  (Exxon)  Heavy aromatic naphtha                                                                       8.7   183   <10                                       DXE                                                                           (Gulf)   1,1-di(ortho-xylyl)ethane                                                                    8.8   335   v.low                                     Han      Aromatic middle                                                      (Exxon)  distillate extract                                                                           8.9   169   <10                                       DOP      dioctylphthalate                                                                             8.9   384   ˜0                                  __________________________________________________________________________     .sup.a Relative evaporation rates where butyl acetate = 100              

Pressurization is required for the erasable inks of the presentinvention which are too viscous to flow at a sufficient rate under theforce of gravity alone. Prior art erasable inks often require 10 psi toproduce an acceptable written line. In contrast, the inks of the presentinvention typically require less than 1.0 psi to produce a written lineof similar quality. The amount of pressure required depends on severalfactors including temperature, the particle size of the pigment, thepolymer employed, and the gap between the ball and the lip of theball-retaining socket. The pressure for any particular combination isreadily determined by one skilled in the art. Various means for applyingappropriate pressure are well known in the prior art including U.S. Pat.No. 3,000,354; U.S. Pat. No. 3,099,252; and U.S. Pat. No. 3,425,779, allincorporated herein by reference.

A preferred additive to the ink of the present invention is polybuteneto enhance the cohesive strength of particularlystyrene-isoprene-styrene elastomers. The polybutene has no effect onpressurization or line intensity, but increases the erasability of inkswhen used in concentrations up to about 10% by weight. Polybutenes are aseries of butylene polymers composed predominently of high molecularweight mono-olefins (85-98%), the balance being isoparaffins. Preferredpolybutenes have a viscosity of about 3026-3381 centistokes at 210° F.and average molecular weight of about 2060. Polybutenes are made bypolymerizing an isobutylene-rich butene stream with a metal halidecatalyst. The polymer backbone structure resembles polyisobutylene,although more 1- and 2-butenes are incorporated in the lowermolecular-weight fractions. The olefin structure is predominently thetrisubstituted type (R--CH═CR₂). Only minor amounts of vinylidene##STR1## and terminal vinyl (R--CH═CH₂) structures are present. Apreferred polybutene is sold under the trade name Indopol H-1500 byAmoco Chemicals Corporation, 200 E. Randolph Dr., Chicago, Ill. 60601.

Another preferred additive is poly-alpha-methylstyrene which can be usedalone or in conjunction with polybutenes. With this additive, lineintensity is unchanged, but erasability is increased with increasingconcentration. However, the pressure to write increases proportionatelywith concentration such that poly-alpha-methylstyrene is useful only inpressurized inks and is typically used in concentrations up to about 10%by weight.

It has been found that the addition of polybutene andpoly-alpha-methylstyrene dramatically reduces the tendency to smearingof prior art erasable inks which is seen initially after writing.

Erasability can be evaluated by manual erasure and visual observation,but this involves personal factors such as pressure applied, size ofsurface being erased, etc. A more accurate evaluation can be conductedby applying the trace by a standard writing test method in which thepaper is advanced beneath the writing instrument at a rate of 11.5 mm.per 10 cycles and at the same rate under an erasing head provided withan Eberhard-Faber Pink Pearl Pencil Eraser #101 (Shore A34 Durometerhardness, ASTM D 2240), applied to the paper bearing the trace under aload of 375 grams upon a surface measuring 9 mm. by 23.8 mm. in thedirection of travel, the erasing head reciprocating at a rate of 80cycles per minute, the length of each stroke being 63.5 mm. Each traceis thus subjected to 7 cycles of erasing and results can be visuallyobserved or rated by photometer readings.

Smearing can be tested by taking Ross micro wax #1365, obtained fromFrank B. Ross Company, Inc., 6-10 Ash St., Jersey City, N.J. and formingit into plugs the size of a standard eraser, placing it into an erasingmachine as described above and running over a fresh tracing five cycles.This produces a reasonably uniform smear which can then be placed into areflectometer and measured against the reflectance of the same paperwithout ink on it.

The equation used to determine the percent smearing is ##EQU2## whereR_(u) is the reflectance of the unmarked paper, R_(m) is the reflectanceof the marked paper, and R_(S) is the reflectance of the smeared area.

While the major components of the ink have been stressed above, thepresence in the ink of additives such as corrosion inhibitors,lubricants such as lauric acid or stearic acid, preservatives, partingcompounds (between ink in a reservoir and a gaseous pressurizing agent),and dispersing agents, is not precluded.

The invention will be further illustrated by consideration of thefollowing examples which are intended to be purely exemplary of the useof the invention.

EXAMPLE I

An ink employing a radial styrene-isoprene-styrene block copolymer soldunder the trademark Solprene 418 by Phillips Chemical Company, Borger,Tex., having a styrene/isoprene ratio of 15/85 was used to prepare anerasable ink requiring no pressurization. 20 grams of the blockcopolymer was dissolved in 49.8 grams of dodecane and 10.2 grams ofdixylylethane. To this was added 20 grams of a pigment dispersion of31.6% Victoria Blue, 14.1% Phthalo Blue, and 54.2% dioctyl phthalate.The combination of solvents produced a solubility parameter of 8.1. Theresulting ink produced a written trace with acceptable line intensityand erasability.

EXAMPLE II

Example I was repeated with 10 grams of a linear block copolymer havinga styrene/isoprene ratio of 14/86 sold by Shell Chemical Company,Houston, Tex., and a mixed solvent system of 67.2 grams of isobutylisobutyrate having a solubility parameter of 8.1, 2.8 grams of methylethyl ketone having a solubility parameter of 9.3, and 20 grams of thepigment dispersion of Example I. The resulting combination of solventshad a solubility parameter of 8.7. The resulting ink produced a writtentrace with acceptable line intensity and erasability.

EXAMPLE III

An ink employing a styrene-ethylene/butylene-styrene block copolymersold under the trademark Kraton G1650 by Shell Chemical Company,Houston, Tex., having a styrene to rubber ratio of about 28 to 72 wasprepared by mixing 10% by weight of block copolymer with 50% by weightof toluene. The dissolved polymer was then mixed with a pigmentdispersion consisting of 60% by weight of toluene, 21.7% dioctylphthalate, 12.6% Victoria Blue and 5.7% Phthalocyanine Blue. Theresulting ink was placed in a conventional refill cartridge andpressurized to a pressure of 0.6 psi. The resulting ink produced awritten trace with acceptable line intensity and moderate erasability.

EXAMPLE IV

Example III was repeated with a styrene-isoprene-styrene block copolymersold under the trademark Kraton 1101 by Shell Chemical Company, Houston,Tex., having a styrene to rubber ratio of 30 to 70 in place of thestyrene-ethylene/butylene-styrene block copolymer. The resulting inkproduced a written trace with acceptable line intensity and gooderasability.

EXAMPLE V

A refill capable of writing without pressurization was preparedemploying an ink containing the following ingredients by weight: 40%Indofast Violet; 5.0% Kraton 1107, a linear styrene-isoprene-styreneblock copolymer having a styrene/isoprene ratio of 14/86 sold by ShellChemical Company, Houston, Tex.; 15.0% Solprene 423, a radialstyrene-isoprene-styrene block copolymer having a styrene/isoprene ratioof 15/85; 5.0% Indopol H 1500, a polybutene sold by Amoco Chemical Co.,Chicago, Ill.; 6% dioctyl phthalate; and a mixed solvent of 49%1,1-di(orthoxylyl)ethane and 16% MagieSol 44 to produce a δ ofapproximately 8.4. The resulting ink produced a written line of goodline intensity and good erasability.

EXAMPLE VI

A refill requiring a pressurization of about 0.3 psi and having goodline intensity and good erasability was produced by the followingformulation:

    ______________________________________                                        Ingredients               %, w/w                                              ______________________________________                                        Kraton 1107                       5.0                                         Solprene 423                      15.0                                        Indopol 1500                      5.0                                         Poly-alpha-methylstyrene          6.0                                         Aromatic 150                                                                                          q.s. to δ =                                                                       8.4                                         MagieSol 44                                                                   Victoria Blue                     15.0                                        Dioctyl phthalate                 15.0                                        ______________________________________                                    

EXAMPLE VII AND VIII

Examples VII and VIII illustrate the beneficial effect of polybutene andpoly-alpha-methylstyrene on smearing tendency.

    ______________________________________                                        INK COMPOSITION (percent by weight)                                                       Example VII                                                                            Example VIII                                             ______________________________________                                        Solprene 423  16.2       14.7                                                 Kraton 1107   5.4        4.9                                                  Indopol H-1500                                                                              --         3.0                                                  Resin 18-210 (1)                                                                            --         6.0                                                  Lauric Acid   1.4        1.1                                                  Stearic Acid  0.5        0.4                                                  Toluene       26.5       19.5                                                 Dioctyl phthalate                                                                           27.1       27.1                                                 BT-264D (2)   15.8       15.8                                                 BT-427D (3)   7.1        7.1                                                  ______________________________________                                         (1) Polyalpha-methylstyrene sold by Amoco Chemicals Corp., Chicago,           Illinois.                                                                     (2) Victoria Blue sold by E. I. DuPont, Wilmington, Delaware.                 (3) Phthalocyanine Blue sold by E. I. DuPont, Wilmington, Delaware.      

The inks of Examples VII and VIII were compared using the smearing testdescribed above. The smearing tendency of the ink of Example VIII wasabout two-thirds that of Example VII and the inks of the prior art.

What is claimed is:
 1. An erasable ink for use in a ball-point writinginstrument comprising a pigmented organic solvent solution containing amixture of thermoplastic block copolymers at least one of which has anarrangement of A (thermoplastic) and B (rubbery) blocks selected fromthe class consisting of ABA and (--AB)--_(n) ; at least one other ofwhich has an arrangement of A (thermoplastic) and B (rubbery) blocksconsisting of (AB)_(n) X; in which said thermoplastic blocks areselected from the class consisting of styrene, ester and urethaneblocks; in which said rubbery blocks are selected from the classconsisting of butadiene, isoprene and ethylene-butylene blocks; in whichthe ratio of said rubbery blocks to said thermoplastic blocks is from60:40 to 90:10; and in which the solubility parameter of said organicsolvent is from about 7 to about 10.5.
 2. An erasable ink as describedin claim 1 in which the ratio of said block copolymers having anarrangement of A (thermoplastic) and B (rubbery) blocks selected fromthe class consisting of ABA and (--AB)--_(n) to said block copolymerhaving an arrangement of A (thermoplastic) and B (rubbery) blocksconsisting of (AB)_(n) X is from about 1:1 to about 1:7.
 3. An erasableink as described in claim 1 in which the ratio of said block copolymershaving an arrangement of A (thermoplastic) and B (rubbery) blocksselected from the class consisting of ABA and (--AB)--n to said blockcopolymer having an arrangement of A (thermoplastic) and B (rubbery)blocks consisting of (AB)_(n) X is from about 1:1 to about 1:3.
 4. Anerasable ink as described in claim 1 in which at least one of said blockcopolymers has butadiene or isoprene midblocks.
 5. An erasable ink asdescribed in claim 1 in which at least one of said block copolymers hasbutadiene or isoprene midblocks and in which the solubility parameter ofsaid organic solvent ranges from about 8.1 to about 8.7.
 6. An erasableink as described in claim 1 in which at least one of said blockcopolymers has butadiene or isoprene midblocks and in which said organicsolvent is a mixture of at least two organic solvents, at least one ofwhich has a solubility parameter of about 7.2 to about 7.8, at least oneother of which has a solubility parameter of about 8.7 to about 8.9 andin which said mixture has a solubility parameter of about 8.1 to about8.7.
 7. An erasable ink as described in claim 1 in which at least one ofsaid block copolymers has butadiene or isoprene midblocks, in which thesolubility parameter of said organic solvent ranges from about 8.1 toabout 8.7 and in which the concentration of pigment does not exceedabout 10% by weight.